Heavy machinery, such as off-highway trucking equipment, is commonly used in mining, heavy construction, quarrying, and other applications. Due to the substantial capital investment involved, tight tolerances with respect to the time allotted for completing tasks, and the expense of maintaining and operating heavy machinery, such as a mining truck, an entity can suffer significant monetary losses when the heavy machinery malfunctions. The complexity of modern heavy machinery often exacerbates this problem due to the need for skilled personnel to perform various tests on such machinery to trouble shoot such malfunctions. Even so, significant time is often spent locating the fault and then performing an appropriate repair.
One advance that has improved efficiency associated with the use of heavy machinery is the adoption of Alternating Current (AC) or electric drive systems. Electric drive systems typically require less maintenance and thus, have lower life cycle costs. When the heavy machinery malfunctions, however, the costs associated with the downtime are often substantial.
Direct series electric drive systems for machines typically include a power circuit that selectively activates one or more drive motors at a desired torque. Each of the drive motors is connected to a wheel or other traction device that operates to propel the machine. A direct series drive system also includes a prime mover, for example, an internal combustion engine, that drives a power generator. The power generator produces electrical power conditioned by a power circuit, and ultimately used to drive the motor. Conceptually, mechanical power produced by the internal combustion engine is converted into electrical power by the generator. The electrical power is processed and/or conditioned by the power circuit before it is metered to the motors. The motors transform the electrical power back into mechanical power that drives the wheels and propel the machine.
The electrical components are susceptible to overheating under high load conditions. In such situations, known systems detect the excessive heat, and a warning light informs the operator. What typically follows is the operator shutting down the machinery to prevent permanent damage to the generator providing electricity to the drive motors and/or the motors driving the machinery.
Another heat management approach incorporated into electrical drive systems, described in Vanek et al., U.S. Pat. No. 5,446,362 involves derating electrical motor operation when a temperature exceeds a threshold value specified by a protection system. Derating of the electrical motor is accomplished by reducing a torque limit. The lowering of the torque limit is carried out by reducing the input to the electrical motor from a power inverter that energizes the windings of the electrical motor.